The present invention relates to arrangements for performing machining operations on tubular objects in general, and more particularly to an arrangement for performing facing and/or chamfering operations at the end face of a pipe.
There are already known arrangements of the above type in which a clampingly holding unit holds the respective pipe in position relative to a support during the chamfering and/or facing operation performed at the end face of the pipe. The support or machine frame of this arrangement also carries a tool holder for movement in the axial direction of the pipe as well as, within a limited range, in the vertical direction. The tool holder is also mounted on the machine frame for rotation about an axis and carries the respective facing or chamfering tools. The tool holder of this conventional arrangement has an internal passage which partially accommodates a push rod or connecting rod having a free end which is connected to a mandrel which is received in the internal passage of the tool holder for movement in the axial direction and centrally into the end of the pipe. The other end of the connecting rod is connected to a cylinder-and-piston unit which is mounted on the machine frame.
An arrangement of the above type is disclosed in the published German patent application DE-OS No. 25 49 671. In this arrangement, the mandrel which is introducible into the respective pipe end is constructed as a calibrating mandrel. This calibrating mandrel has a tapering surface at that end thereof which is closer to the pipe, and is pressed by the cylinder-and-piston unit into the open end of the pipe with the express aim to eliminate any deviations from circularity which the respective pipe end may possess. In order to be able to press this calibrating mandrel into the open end of the respective pipe, it is displaceable in the vertical direction with respect to the axis of the cylinder-and-piston unit and is supported on the machine frame by means of springs. However, this calibrating mandrel is laterally guided in a positive manner. In this manner, the calibrating mandrel can become aligned with the pipe in the vertical direction, but not in the horizontal direction. The result of this is that an exact alignment of the pipe axis with the axis of the connecting rod that guides the tool carrier is impossible. More particularly, it must be taken into consideration in this connection that the pipe axis related to the outer diameter of the pipe does not necessarily coincide with the pipe axis related to the inner diameter of the pipe. When, under these circumstances, the tool carrier which is axially movably guided on the connecting rod is moved toward the end face of the respective pipe end portion, it is impossible to produce by machining either the finished end face which would extend exactly normal to the axis of the pipe as related to the inner diameter of the pipe, or chamfered surfaces which would lie within the permissible tolerance range. It is especially impossible to guarantee that the end faces and the chamfered surfaces are so configurated that, during the welding of two pipes to one another, there exists an uninterrupted and especially step-less transition between the inner surfaces of such joined pipes. Yet, in many types of practical applications, there is required an unobjectionable coincidence of the pipe axes of the consecutive pipes as related to the inner diameters of such pipes with the abutting surfaces of these pipes extending normal to such axes. This is a requirement in some applications whether the pipes are to be joined by welding or by a threaded connection.
In order to satisfy such requirements, it was previously proposed to so center the axis of rotation of the tool holder on the axis of the pipe as related to the inner pipe diameter that the inner surface of the pipe is sensed by means of sensors in the form of contacting rollers or copying rollers. An important disadvantage of this approach is that it is not suited for use with pipes having an inner diameter smaller than approximately 40 millimeters. Another disadvantage is that one of the copying rollers may, for instance, roll over fitting chips or similar irregularities, so that it necessarily yields away from the actual internal pipe surface and, as a result of this, the required tolerances cannot be achieved any longer. In addition, it is disadvantageous in this context that a copying head of this kind must be frequently maintained or repaired, in that it consists of a multitude of complicated parts which, additionally, are very wear prone, because of their constant relative movements.
Thus, it is impossible to achieve, with the conventional facing and/or chamfering arrangements, the exactly planar configuration of the end faces of pipes extending in a plane precisely normal to the axis of the respective pipes as related to the inner diameter of the respective pipe, as it is required for the welding of the end portions of consecutive pipes to one another or for the provision of unobjectionable threads on the pipe end portions, and/or the chamfered surfaces within the required range of tolerances.